The present invention relates generally to isolation of conductive windings. It finds particular application in conjunction with low-profile planar magnetic devices such as transformers and inductors constructed to meet existing isolation standards. Use of isolated windings produced according to the present invention allows construction of smaller sized low-profile planar transformers and inductors capable of handling voltages previously handled by larger devices. It is to be appreciated, however, that the invention has broader application and may be employed in other environments.
As the size of electronic components have decreased the size of devices incorporating those components have correspondingly decreased. Regulated switching power supplies have, for example, particularly benefitted from such advancements. These power supplies are well known for their high efficiency, cool operation, small size, and ability to work with a wider range of input voltages than their linear counterparts. As the size of the power supplies have decreased the miniaturization of the magnetic devices used in conjunction with the power supplies have also become a requirement.
In designing planar magnetic devices, stringent isolation requirements which meet safety standards developed by recognized safety agencies must be met. Isolation, however, is largely an issue of separation and insulation between conductors and connections. This separation and insulation works against size reduction. Therefore, a trade-off exists between the miniaturization desired and the linear distance needed to meet existing isolation requirements.
While the desire to minimize the distances between the conductive elements in low-profile planar magnetic devices and thereby reduce the overall size of the devices exists, there has been an inability to obtain desired size reductions and also satisfy the isolation safety requirements. This in turn has limited the use of designs which provide for small, light weight, low-profile planar magnetics such as transformers and inductors. Therefore, the challenge facing designers of low-profile planar magnetics is to meet the isolation safety requirements and yet construct miniaturized devices which can be incorporated into power supplies and other electronic devices which themselves have been down-sized.
As the power supply industry has evolved into a global market the safety agencies for this market have harmonized their requirements. The safety agencies in North America, Underwriters' Laboratories in the United States and the Canadian Standards Association in Canada along with the European Community countries have based their requirements on those established by the International Electro-Technical Commission (IEC). The Information Processing Equipment, IEC 950, standard has been used extensively for harmonizing the standards applied to evaluate equipment that provides isolation from the AC mains and operators or users of equipment. This standard requires the use of reinforced isolation between the AC mains and user accessible terminals.
Reinforced isolation requirements can be met by using any one of the following methods for circuits operating from mains up to 250 VAC: 1) through air spacing of 4 mm (0.16 inch); 2) over surface spacing of 8 mm (0.32 inch), this is a worst case spacing and may be reduced to 5 mm (0.20 inch) for controlled environment applications; 3) three layers of insulation (no minimum thickness requirements), with any combination of two layers supporting the dielectric test levels required for the circuit; 4) solid insulation with a minimum thickness of 0.4 mm (0.016 inch) void free (no air bubbles); and 5) over surface spacing which may be reduced to 1.2 mm (0.048 inch) when a suitable conformal coating that provides a minimum of 80% coverage of the space between conductors is used. The aforementioned isolation methods are tested to safety agency requirements which include temperature cycling, humidity testing and dielectric testing to assure compliance.
U.S. Pat. No. 5,010,314 to Estrov addresses the challenge facing low-profile planar transformer designs. Estrov ('314) attempts to meet the safety agency requirements through the use of a bobbin design which is incorporated into a sandwich-like-laminent of dielectric insulators, spacers, windings and bobbins enclosed by a magnetic housing made of a core material. While some decrease in size may be obtained by such a construction the edges of the Estrov device are open ended, i.e. unsealed, and a bobbin is maintained within the transformer design. Constructing a device with open edges, and maintaining the bobbin, along with the other required elements results in a transformer having dimensions which are unacceptably large for incorporation into designs such as those found in circuit board layouts.
It has, therefore, been considered desirable to provide isolated conductive windings with sealed bonded edges constructed according to the present invention. These windings can then be used in low-profile planar magnetic devices that eliminate the use of bobbins and which meet existing safety requirements, in a reduced physical size. Such isolated windings may be used in low-profile planar transformers or inductors reducing their overall size, resulting in smaller transformers and inductors which can nevertheless handle voltages previously handled by larger sized devices. Such devices should be economical to manufacture and be of a sturdy overall construction. The subject invention is deemed to meet the foregoing needs and others.